The invention relates to a method and for producing an aerogel on a substrate. Aerogels are highly porous materials comprising silicon or metal oxides, which are characterised by particularly low densities of 70-300 Kg/m3 at extremely high internal surface areas of up to 1000 m2/g (see DE 3924244 A1). They are produced from a lyogel, which is produced in a sol-gel process from a lyosol. Gels are shape-stable disperse systems comprising at least two components, generally a solid, colloidally distributed material with long or heavily branched particles and a dispersant. If the dispersant between the particles is a liquid, a lyogel is present. If the liquid of the lyogel is replaced by air as the dispersant, an aerogel is produced.
A known method for producing a SiO2 aerogel is described S. S. Kistler in J. Phys. Chem. 36 (1932), pages 52-64. Water glass is used therein as the precursor and acid (HCl; H2SO4). A hydrogel is formed in which the water is subsequently substituted by ethanol or methanol. The alcohol-containing gel which is produced is then subjected in an autoclave to a pressure of more than 71 bar and a temperature of  greater than 100xc2x0 C. The solvent is in a state above the critical point at which there is no more surface tension. The solvent can escape from the gel without shrinking of the gel occurring by reason of the surface tension. The escaping solvent is drawn off so that drying of the gel occurs. An aerogel is produced. Of disadvantage with this so-called supercritical drying in an autoclave are the high pressures and temperatures which result in an expensive method. Methods are also known in which the alcohol is replaced by carbon dioxide, which permits supercritical drying at low temperatures. Furthermore, two further methods are mentioned in International Patent Publication WO92/926213 A1. A first method mentioned therein is freeze drying. An alcogel (ie. a lyogel with alcohol as the dispersant) is frozen so that a solid gel is produced. The solvent is subsequently removed under reduced pressure by sublimation. The publication refers to no solvent suitable for this purpose and, furthermore, condemns this method due to a disadvantageous volumetric increase during crystallisation or during freezing. Furthermore, the publication is concerned with a method for directly vaporising the solvent from the liquid phase into the gas phase. The negative influences of the surface tension are to be minimised by a suitable solvent mixture and conduct of the method.
A method is also disclosed in publication WO95/17347 A1 which, for the purpose of avoiding the disadvantages of the supercritical drying, proposes a sub-critical vaporisation at increased or reduced pressure with respect to normal pressure or at ambient pressure. The disadvantageous effect of the surface tension of the vaporising liquid is reduced in this case also by a suitable conduct of the method and/or suitable solvents, which exhibit a low surface tension. Although the publication names a large number of possible solvents on an inclusive basis, only embodiments with the solvent isoproponol, optionally with the addition of methanol, are described in more detail.
A method of producing aerogels, which is also supposed to find application in the field of microelectronics, is disclosed in EP 0775669 A2. A precursor comprising tetraerthoxysilane (TEOS) water and multisolvent (a solvent mixture comprising a polyol and ethanol) is used. After deposition of the gel by spinning it onto a semi-conductor wafer, the ethanol is vaporised. After addition of a catalyst from the gas phase, gelling occurs, whereby a wet SiO2 network is produced, the gel is then dried by vaporisation of the polyol. As a result of a reaction between the polyol and TEOS, the storage and transport time of the precursor is heavily limited. The addition of the catalyst from the gas phase results in a vertical diffusion gradient in the aerogel layer, whereby inhomogeneous layer properties can occur in the vertical direction.
It is an object of the invention to produce an aerogel layer with homogeneous properties in a simple and economical manner.
This and other objects of the present invention are therein solved by a method of producing an aerogel layer on a substrate, said method comprising the steps of: a) providing a precursor mixture by mixing at least one material from a group including silicates, metal alcoholates, aluminates and borates with a solvent, said precursor mixture for forming a lyosol; b) applying the precursor mixture or a lyosol formed therefrom to said substrate, wherein a layer of the lyasol is formed on said substrate; c) forming a gel from the lyosol, a temperature being selected at which the solvent is present in a liquid state; d) reducing, at a pressure of between 0.5 and 2 bar, the temperature by about 3-70K below the point at which the solvent is converted into the solid state; and e) converting the solvent into the gaseous state in a drying chamber whilst reducing the pressure of the solvent below the triple point and removing the solvent from the gel layer.
In the method in accordance with the invention for producing an aerogel layer on a substrate, a precursor is firstly prepared by mixing at least one material from a group including silicates, metal alcoholates, aluminates and borates with a solvent to form a lyosol mixture. Silicates include the salts and esters of orthosilicic acid and their condensation products, that is to say for instance, the silicic acids and the silicon alcoholates (also referred to as silicon alcoxides or alcoxysilanes). A lyosol is a colloidal solution in which a solid or liquid substance is dispersed in a liquid medium. The addition of water can possible be necessary to form the lyosol. The lyosol is not produced directly on mixing the material with the solvent but only after an initial chemical conversion (e.g. hydrolysis and polycondensation).
The precursor mixture or the lyosol formed therefrom is then applied to the substrate. As a result of further chemical conversion (hydrolysis and polycondensation), a gel is formed from the lyosol, whereby a temperature is selected at which the solvent is present in a liquid state. The pressure is preferably so selected that it is between 0.5 and 2 bar where the range is include, ambient pressure and a slightly reduced pressure. The pressure range may be adjusted relatively easily from the process engineering point of view. The temperature is so selected that the solvent is present in a liquid state in order to promote the formation of a gel.
After the has been formed (whereby small accounts of the starting materials and products of the reactions can remain in the gel), the temperature is reduced, at a pressure between 0.5 and 2 bar, by about 2-70K, preferably 5-15K, below the point at which the solvent changes into the solid state (solidifies). The only slight reduction of temperature below the solidification point results, on the one hand, in a saving of energy and an acceleration of the method and, on the other hand, in low mechanical stresses in the gel layer induced by the temperature and solidification.
The solvent is then converted into the gaseous state in the drying chamber whilst reducing the pressure of the solvent to below the triple point and removed from the gel layer. The gaseous solvent is conducted out of the drying chamber. When reducing the pressure, the temperature can be maintained constant or increased. Of importance is merely that the change of state from the solid into the gaseous state occurs without passing through the liquid state, ie. below the triple point.
Preferably a solvent is used which has a triple point at temperature above 0xc2x0 C., preferably above 15xc2x0 C. This permits a considerable simplification of the apparatus since cooling water can be used for cooling purposes.
In an advantageous embodiment of the method in accordance with the invention, a low-molecular tertiary alcohol is used as the solvent. Low-molecular tertiary alcohols, particular t-butanol, have, on the one hand, a triple point at a relatively high temperature. On the other hand, tertiary alcohols are less reactive and have a smaller tendency to alcoholysis. They thus have a better compatibility with a large proportion of the group of materials including the silicates, which results in better transportability and storage ability of the precursor mixture. Furthermore, their evaporation properties permit rapid sublimation.
In a preferred embodiment of the method in accordance with the invention a precursor is prepared by mixing a material predominantly comprising an alcoxysilane with a low-molecular tertiary alcohol, constituting the solvent, and with water. An aerogel substantially comprising silicon oxide is formed from such a precursor. In addition to the predominant proportion of the alcoxysilane, the mixture can include, for instance, a small proportion of a metal alcoholate or borate in order to modify the silicon oxide structure. The addition of water serves to hydrolyse the alcoxysilane, whereby the products which are produced cross-link by polycondensation. Although water is formed again during the latter, a larger amount is consumed during the hydrolysis.
Tetraethoxysilane (TEOS) is preferably used. T-butanol is preferably used as the low-molecular tertiary alcohol. Tetraethoxysilane is an economic starting material, which is frequently used within semi-conductor technology and which furthermore, is not so toxic as eg. tetramethoxysilane (TMOS). It has transpired that the combination of tetraethoxysilane with t-butanol results in a precursor with an excellent storage ability and outstanding processing qualities, associated with which, amongst other things, is a relatively small health risk. The combination of materials also has a good miscibility with water, which is required as a reaction partner for the hydrolysis of the tetraethoxysilane.
The water, which is mixed into the precursor in addition to the solvent in the preferred embodiment, acts principally as a reaction partner for the hydrolysis as a prerequisite for the sol-gel method. The precursor is preferably prepared by firstly mixing a material from the group of materials referred to above with the solvent, whereby the solvent (possibly solid at room temperature) is liquefied by heating before the mixing. The water is then mixed in. A catalyst is preferably added to accelerate the gel formation. Both multi-stage catalysis methods, in which, initially, a first catalyst (eg. NH4OH) which promotes the hydrolysis and, subsequently a second catalysis (eg. HCl), which promotes the condensation are added and the addition of only one catalyst (eg. HF), which accelerates both reactions, are possible. The use of a catalyst, which is preferably added in an aqueous solution, accelerates The method and thus increases its efficiency.
A preferred embodiment of the method in accordance with the invention is characterised in that a pre-gelling occurs before the application of the lyosol to the substrate. This increases the viscosity of the substance so that relatively thick layers of the gel can be produced with simple, conventional application methods, for instance, by spinning.
In a preferred embodiment of the method in accordance with the invention, the gel layer formed on the substrate is washed with the solvent before the reduction in the temperature to solidify the solvent. This serves to enrich the solvent in the gel layer and, at the sane time, to remove excess starting materials of the hydrolysis and condensation reactions. A hydrophobic agent can advantageously be added at the same time to the washing process, which replaces OH groups curd water in the gel layer by alkyl groups (eg. hexamethyldisilazane or trimethylchlorosilane).
A preferred embodiment of the method in accordance with the invention is characterised in that a precursor is prepared by mixing tetraethoxysilane with more than twice the amount (moles) of t-butanol, preferably with 4 to 30 times the amount of t-butanol, at a temperature at which the mixture is present in liquid form and water is added in an amount which corresponds to 4 to 30 times, preferably about 6 to 10 times the amount of the tetraethoxysilane. A catalyst which accelerates hydrolysis and/or condensation is then added. Before application to the substrate, the mixture is then stored at a temperature at which the mixture reimains liquid and under an atmosphere saturated with t-butanol. The pre-gellification takes place which increases the viscosity. Depending on the choice of catalyst, the mixture is preferably stored for a period between 1 hour and a number of days. Catalysts are also possible which permit a shorter time. The pre-gelled mixture is then applied to the substrate by spinning and caused to solidify by reducing the temperature. These steps preferably occur at pressures between 0.5 and 1 bar. The t-butanol is finally converted to a gaseous state below the triple point by reducing the pressure to below 0.05 bar and is, thus, removed from the gel layer. The gaseous t-butanol is conducted out of the drying chamber. After the drying process, the aerogel layer applied to the substrate is post-treated at a temperature between 200xc2x0 C. and 800xc2x0 C. in an inert gas atmosphere or a reduced pressure air atmosphere. Both the residue of the water and also residues of undesired excess starting materials are, thus, removed. A treatment with a hydrophobia-imparting agent (e.g. hexamethyldisilazane or trimethylcliorosilane) is then preferably performed in order to replace remaining OH groups (or water) with methyl or other alkyl groups. This treatment can be performed in the gas phase.